JP2010164375A - Concentration measuring instrument and concentration measuring method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a concentration measuring instrument capable of detecting the absorption of a laser beam with higher sensitivity, and a concentration measuring method. <P>SOLUTION: The concentration measuring instrument is equipped with an emission control part for controlling a laser diode, which emits the laser beam to a measuring target so that the wavelength of the laser beam may be modulated by a first modulation wave and a light reception processing part for acquiring the light reception signal from a photodiode receiving the laser beam transmitted through the measuring target and measuring the concentration of the measuring target on the basis of the light reception signal. The first modulation wave has a waveform wherein the absolute value of inclination at the time becoming zero in intensity is smaller than that of a sine wave having the same amplitude and the same frequency. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to a concentration measuring device and a concentration measuring method.

  As a technique for measuring the concentration of a measurement target, a technique using laser light is known. In this technique, it is utilized that a substance has a property of absorbing light having a wavelength (absorption wavelength) unique to the substance. Irradiate the measurement target with laser light. If the wavelength of the laser beam matches the absorption wavelength of the measurement target substance, the laser beam is absorbed by the measurement target. The amount of absorption at this time depends on the concentration of the substance. Therefore, the laser beam transmitted through the measurement target is received, and the amount of absorption of the laser beam by the measurement target is obtained. Based on the obtained absorption amount, the concentration of the measurement target can be calculated.

  In measuring the concentration of the measurement target, it is desired to improve the measurement accuracy. In order to improve accuracy, a method of modulating the wavelength of laser light is known. The wavelength of the laser light is modulated by a modulated wave having a predetermined frequency. Then, the received light signal obtained on the light receiving side is demodulated based on the frequency of the modulated wave. Thereby, the noise component is removed, and the amount of absorption of the laser beam by the measurement target can be accurately known.

  Techniques for further improving the accuracy are described in Patent Document 1 (Patent No. 3865959) and Patent Document 2 (Patent No. 3342446). Patent Document 1 and Patent Document 2 describe that the oscillation wavelength of laser light is modulated at at least two different frequencies, and the modulated signal among the received signals is sequentially demodulated for each frequency. According to the techniques described in Patent Documents 1 and 2, the influence of fringes generated in optical parts such as mirrors can be removed by modulating laser light at least twice, and the measurement accuracy is further improved. Can do.

  A sine wave is usually used as a modulation wave when modulating the wavelength of the laser beam.

Japanese Patent No. 3861059 Japanese Patent No. 3342446

  FIG. 1 is an explanatory diagram for explaining the waveform of the amount of absorption of laser light when the wavelength of the laser light is modulated with a sine wave. FIG. 1A is a graph showing the relationship between the amount of light absorbed by the measurement target substance and the wavelength. FIG. 1B shows the waveform of the modulated wave and shows the relationship between the time t and the wavelength of the laser beam. FIG. 1C is a graph showing the relationship between the amount of laser light absorbed by the measurement target and time t.

As shown in FIG. 1A, in many substances, the wavelength distribution of the light absorption amount is a Gaussian distribution centered on the central wavelength λ ₀ . Here, as shown in FIG. 1B, it is assumed that the wavelength of the laser light is modulated by a sine wave having a period of 2T with λ ₀ as the center. Then, as shown in FIG. 1C, the absorption amount of the laser light shows a waveform of the period T. In other words, if the frequency of the modulated wave is f, the amount of laser beam absorption is represented by a 2 × f component. This is because the amount of light absorption shows a Gaussian distribution with respect to the wavelength. By utilizing this and demodulating the laser light absorption amount waveform with 2 × f components, it is possible to extract information on the laser light absorption amount by eliminating the influence of noise. As a result, it is possible to accurately measure the concentration.

However, in the above-described method, the center value of the wavelength of the modulated laser light, must match the center wavelength lambda _0. If the center value of the modulation wave is deviated from the center wavelength λ ₀ , the amplitude of the absorption waveform of the laser beam is also deviated, and accurate measurement cannot be performed.

Therefore, there is a case in which a carrier wave that always passes through the center wavelength λ ₀ is used and the carrier wave is modulated with a sine wave. FIG. 2 is a schematic diagram showing the relationship between time and the wavelength of laser light when a ramp wave is used as a carrier wave. At time t _{0 when} the wavelength of the laser beam passes λ ₀ , the laser beam is absorbed by the measurement target. FIG. 3 is a schematic diagram showing a waveform of the amount of absorption of laser light when the laser light having the waveform shown in FIG. 2 is used. The amplitude of the absorption amount of the laser light is around the time t _0, indicating a Gaussian distribution. Therefore, the 2 × f component is phase-detected using a lock-in amplifier or the like from the signal indicating the amount of absorption of the laser light, and the low-frequency component is extracted using a low-pass filter or the like. In FIG. 3, the extracted waveform of the low-frequency component is indicated by a dotted line. The extracted peak value A1 of the low-frequency component corresponds to the amount of laser light absorbed at time t0. That is, it corresponds to the amount of laser light absorbed at the center wavelength λ ₀ . Therefore, by obtaining the peak value A1, the amount of absorption of the laser beam at the center wavelength λ ₀ can be obtained. Thereby, it is possible to calculate the density of the measurement target. 2 and 3 exemplify the ramp wave as the carrier wave, the ramp wave is merely an example, and other waveforms may be used as long as the waveform passes the absorption wavelength to be measured.

  By the way, as shown in FIG. 3, the peak value A1 is smaller than the amplitude A2 of the absorption amount of the laser beam. If the peak value A1 can be increased, it is considered that the amount of absorption of laser light can be detected with high sensitivity, and the influence of noise can be more reliably eliminated.

  Accordingly, an object of the present invention is to provide a concentration measuring device and a concentration measuring method capable of detecting absorption of laser light with higher sensitivity.

  In the following, means for solving the problem will be described using the numbers used in [Best Mode for Carrying Out the Invention] in parentheses. These numbers are added to clarify the correspondence between the description of [Claims] and [Best Mode for Carrying Out the Invention]. However, these numbers should not be used to interpret the technical scope of the invention described in [Claims].

  The concentration measuring apparatus (1) according to the present invention includes a laser diode (4) that emits laser light toward a measurement target, and a laser diode (4) so that the wavelength of the laser light is modulated by a first modulated wave. ), A photodiode (6) that receives a laser beam transmitted through the measurement object (5) and generates a light reception signal, and a measurement object (5) based on the light reception signal. And a light receiving processing unit (3) for measuring the density of the light. The first modulated wave is a waveform having a smaller absolute value of the gradient when the intensity becomes zero than a sine wave having the same amplitude.

According to the present invention, when the center of the first modulated wave coincides with the center λ ₀ of the absorption wavelength to be measured, the period during which the wavelength of the laser light passes in the vicinity of the center λ ₀ is longer than when the center is the sine wave. become longer.
In the vicinity of the center λ _0, the amount of laser light absorbed by the measurement target is large. Accordingly, the period during which the amount of absorption is large becomes longer. As a result, the absorption of laser light can be detected with high sensitivity.

  The concentration measuring device (1) according to the present invention controls a laser diode (4) that emits laser light toward a measurement target (5) and the laser diode so that the wavelength of the laser light is modulated by a sine wave. Measures the concentration of the measurement target (5) based on the light emission control unit (2), the photodiode (6) that receives the laser light transmitted through the measurement target (5) and generates a light reception signal, and the light reception signal And a light receiving processing unit (3). The light reception processing unit (3) uses an absorption signal generation unit (12) that generates an absorption signal indicating the amount of laser light absorbed by the measurement target (5) based on the received signal, and the absorption signal using the processing function. And a function processor (20, 21) for converting the waveform into a sine wave and generating a converted absorption signal.

  The concentration measuring method according to the present invention includes a step of controlling a laser diode (4) that emits laser light toward a measurement target (5) so that the wavelength of the laser light is modulated by a first modulated wave, and measurement. Receiving a laser beam transmitted through the object (5) and generating a received light signal; and measuring a concentration of the measurement object based on the received light signal. The first modulated wave is a waveform having a smaller absolute value of the gradient when the intensity becomes zero than a sine wave having the same amplitude.

  ADVANTAGE OF THE INVENTION According to this invention, the density | concentration measuring apparatus and the density | concentration measuring method which can detect absorption of a laser beam with higher sensitivity are provided.

It is explanatory drawing for demonstrating what kind of waveform the absorption amount of a laser beam is represented. It is the schematic which shows the relationship between time and the wavelength of a laser beam. It is the schematic which shows the waveform of the absorption amount of a laser beam. It is a block diagram which shows the density | concentration measuring apparatus which concerns on 1st Embodiment. It is a graph which shows an example of a modulated wave signal. It is a conceptual diagram which shows an absorption waveform. It is a conceptual diagram which shows an absorption waveform. It is a graph which shows the relationship between amplitude coefficient (delta), the amplitude of an absorption waveform, and the abundance ratio of the harmonic component contained in a modulation wave. It is a graph which shows the waveform of (a) modulation wave signal and (b) absorption waveform. It is a graph which shows the waveform of (a) modulation wave signal and (b) absorption waveform. It is a graph which shows the waveform of (a) modulation wave signal and (b) absorption waveform. It is a graph which shows the waveform of (a) modulation wave signal and (b) absorption waveform. It is a graph which shows the waveform of (a) modulation wave signal and (b) absorption waveform. It is a graph which shows the waveform of (a) modulation wave signal and (b) absorption waveform. It is a graph which shows the waveform of (a) modulation wave signal and (b) absorption waveform. It is a graph which shows the waveform of (a) modulation wave signal and (b) absorption waveform. It is a graph which shows the waveform of (a) modulation wave signal and (b) absorption waveform. It is a graph which shows a Fourier-transform analysis result. It is a graph which shows a Fourier-transform analysis result. It is a graph which shows a Fourier-transform analysis result. It is a graph which shows a Fourier-transform analysis result. It is a graph which shows a Fourier-transform analysis result. It is a graph which shows a Fourier-transform analysis result. It is a graph which shows a Fourier-transform analysis result. It is a graph which shows a Fourier-transform analysis result. It is a graph which shows a Fourier-transform analysis result. It is a graph which shows the relationship between amplitude coefficient (delta) and the amplitude of a modulated wave signal. It is a graph which shows a simulation result. It is a graph which shows a simulation result. It is a graph which shows a simulation result. It is a graph which shows the waveform shown by the cube of a sine wave. It is a graph which shows a simulation result. It is a block diagram which shows the density | concentration measuring apparatus which concerns on 2nd Embodiment.

(First embodiment)
Hereinafter, a first embodiment of the present invention will be described with reference to the drawings. FIG. 4 is a block diagram showing the concentration measuring apparatus 1 according to this embodiment.

  The concentration measuring apparatus 1 includes a light emission control unit 2, a laser diode 4, a photodiode 6, and a light reception processing unit 3. The light emission control unit 2 is a part that controls the laser diode 4. The light emission controller 2 emits laser light having a predetermined wavelength from the laser diode 4. The laser light is guided to the measurement target gas region 5 and is received by the photodiode 6 after being absorbed by the measurement target. The photodiode 6 receiving the laser light generates a light reception signal D and supplies it to the light reception processing unit 3. The light reception processing unit 3 calculates the density of the measurement target based on the light reception signal D.

  The light emission control unit 2 includes a carrier wave generator 7, a first modulated wave generator 8-1, a second modulated wave generator 8-2, an adder 9, and a laser diode driver 10.

  The carrier wave generator 7 generates a ramp wave as the carrier wave signal A.

  The first modulated wave generator 8-1 generates the first modulated wave signal B1. The frequency of the first modulated wave signal B1 is sufficiently larger than the frequency of the carrier wave signal A and is f1. f1 is, for example, 100 (kHz).

  The second modulated wave generator 8-2 generates the second modulated wave signal B2 at a frequency f2 that is sufficiently smaller than the first modulated wave signal B1. For example, f2 is 5 (kHz).

  The adder 9 adds the carrier signal A, the first modulated wave signal B1, and the second modulated wave signal B2 to generate a combined signal C.

  The laser diode driver 10 changes the injection current of the laser diode 4 according to the composite signal C.

  The laser diode 4 oscillates laser light in response to the supply of the injection current. Here, the laser diode 4 is configured such that the oscillation wavelength of the laser light changes according to the injection current. The wavelength of the laser light changes with the same waveform as the waveform of the injection current. Since the waveform of the injection current is the same as that of the synthesized signal C, the wavelength of the laser light changes with the same waveform as that of the synthesized signal C. That is, the wavelength of the laser light has a waveform as shown in FIG. However, in FIG. 2, the waveform by the second modulated wave signal B2 is not reflected due to the scale.

  The laser beam emitted from the laser diode 4 is guided to the measurement target gas region 5. When the wavelength of the laser light matches the absorption wavelength specific to the measurement target included in the measurement target gas region 5, the laser light is absorbed by the measurement target. The laser beam that has passed through the measurement target gas region 5 is received by the photodiode 6.

  When the photodiode 6 receives the laser beam, the photodiode 6 generates a light reception signal D indicating the intensity of the laser beam. The light reception signal D is supplied to the light reception processing unit 3.

  The light receiving processing unit 3 includes a photodiode amplifier 11, a differential amplifier 12, a gain amplifier 13, an analog / digital converter 14, a first phase detector 15, a first low-pass filter 16, and a second phase detection. A device 17, a second low-pass filter 18, and a gas concentration calculation unit 19 are provided.

  The photodiode amplifier 11 amplifies the received light signal D and generates an amplified signal E. The amplified signal E is supplied to the differential amplifier 12.

  The differential amplifier 12 differentially amplifies the amplified signal E with the combined signal C. The output signal of the differential amplifier 12 indicates the amount of laser light absorbed in the measurement target gas region 5. The output signal of the differential amplifier 12 is supplied to the gain amplifier 13 as the absorption signal F.

  The absorption signal F is amplified by the gain amplifier 13, converted into a digital signal by the analog / digital converter 14, and sent to the first phase detector 15.

  The first phase detector 15 detects the phase of the transmitted absorption signal F at a frequency of 2 × f1 and generates a first demodulated signal G1. The first demodulated signal G1 is sent to the first low-pass filter 16.

  The first low-pass filter 16 extracts a low frequency component of the first demodulated signal G1, and generates a first low frequency signal H1. The first low frequency signal H <b> 1 is sent to the second phase detector 17.

  The second phase detector 17 detects the phase of the first low frequency signal H1 at a frequency of 2 × f2 and generates a second demodulated signal G2. The second demodulated signal G2 is sent to the second low-pass filter 18.

  The second low-pass filter 18 extracts a low frequency component of the second demodulated signal G2, and generates a second low frequency signal H2. The second low frequency signal H <b> 2 is sent to the gas concentration calculation unit 19.

  The gas concentration calculation unit 19 calculates the peak value of the second low frequency signal H2. The calculated peak value depends on the concentration of the measurement target in the measurement target gas region 5. Therefore, the gas concentration calculation unit 19 calculates the concentration of the measurement target based on the calculated peak value and outputs the calculation result.

  As described above, the influence of fringe can be eliminated and the concentration can be accurately measured by using modulated wave signals (first modulated wave signal B1 and second modulated wave signal B2) having two different frequencies. Is possible.

  Furthermore, in this embodiment, in order to detect the absorption amount of the laser beam by the measurement object with high sensitivity, the waveform of the modulated wave signal (each of the first modulated wave signal B1 and the second modulated wave signal B2) is devised. ing.

  FIG. 5 is a graph illustrating an example of a modulated wave signal according to the present embodiment. In FIG. 5, the horizontal axis indicates time t, and the vertical axis indicates signal intensity. In FIG. 5, the modulated wave signal is indicated by a solid line. For comparison, a dotted line shows a sine wave waveform having the same amplitude and frequency as the modulated wave signal.

  As shown in FIG. 5, the absolute value of the slope of the modulated wave signal when the intensity becomes zero is smaller than that of the sine wave. That is, the inclination when passing through a point where the intensity becomes zero is gentler than that of the sine wave. By using such a modulated wave signal, the light absorption processing unit 3 can detect the absorption amount of the laser light with high sensitivity. The reason for this will be described below.

  FIG. 6 is an explanatory diagram for explaining the waveform of the absorption amount of laser light (hereinafter referred to as an absorption waveform) in the present embodiment. FIG. 6A shows the relationship between the amount of light absorbed by the measurement target and the wavelength. FIG. 6B shows the relationship between the wavelength of the laser beam and time t. FIG.6 (c) shows an absorption waveform, a horizontal axis is time and a vertical axis | shaft is absorption amount. In the graph of FIG. 6C, the waveform of the absorption signal F is obtained by replacing the vertical axis with the signal intensity.

As shown in FIG. 6 (a), the wavelength distribution of the amount of light absorbed by the measurement object, and indicates a Gaussian distribution centered at lambda _0. Consider the case where the oscillation center wavelength of the laser light coincides with λ ₀ as shown in FIG. At this time, the time change of the wavelength of the laser light has a waveform corresponding to the modulated wave signal. When the intensity (wavelength) of the modulated wave signal is zero (wavelength is λ ₀ ), the absorption amount of the laser light has a peak value, and also shows a peak value in the absorption waveform. That is, the peak value of the absorption waveform appears at times T and 2T. Here, the gradient of the modulated wave signal according to the present embodiment is gentle when passing through a point where the intensity is zero. That is, the period in which the wavelength of the laser light is in the vicinity of λ ₀ is long. As a result, in the absorption waveform, the period in which the intensity is close to the peak value becomes long. That is, the absorption waveform shows a wide mountain-shaped waveform. On the other hand, as shown in FIG. 1, the sine wave has a steep slope when passing through a point where the intensity becomes zero. Therefore, the period in which the wavelength of the laser light is in the vicinity of λ ₀ is short. As a result, the absorption waveform becomes a waveform that suddenly drops before and after the peak.

Actually, the center value of the wavelength of the laser light changes linearly by the carrier wave (ramp wave). FIG. 7 is a conceptual diagram showing an absorption waveform when a carrier wave is taken into consideration. In FIG. 7, the absorption waveform is indicated by a solid line. The amplitude of the absorption waveform shows a Gaussian distribution around the time t ₀ when the center value of the wavelength of the laser beam becomes λ ₀ . Here, at least in the vicinity of time t0, a wide mountain-shaped waveform is shown as described above. The absorption signal F corresponding to this absorption waveform is demodulated by a phase detector (first phase detector 15 or second phase detector 17), and further by a low-pass filter (first low-pass filter 16 or second low-pass filter 18). When the low frequency component is extracted, a waveform indicated by a dotted line in FIG. 7 is obtained. Compared to the case where modulation is performed with a sine wave (see FIG. 3), the peak width in the vicinity of time t0 is wide, and therefore the peak value of the extracted low-frequency component is high. Thereby, the absorption of the laser beam can be detected with high sensitivity.

Next, a preferable waveform will be described as the waveform of the modulated wave signal. In order to maximize the output efficiency, it is considered that the absorption waveform may be a sine waveform having no harmonic component in the vicinity of time t ₀ . Therefore, consider the waveform of the modulated wave signal so that the absorption waveform is a sine waveform.

  If the relationship between the amount of light absorbed by the measurement object and the wavelength follows a Gaussian distribution, the amount of light absorption φ (λ) by the measurement object is expressed by the following equation (1).

In the above formula 1, A represents the gas absorption rate at the center wavelength λ ₀ , and λ represents the wavelength.

  On the other hand, the absorption waveform y (t) is expressed by the following Expression 2 assuming that the desired sine waveform is obtained.

  In Equation 2, T represents the period of the absorption waveform, a represents the amplitude of the absorption waveform, δ represents the amplitude coefficient of the absorption waveform, and n represents a natural number (1, 2,...).

  Here, when y (0) = A is considered as the boundary condition, the amplitude a of the absorption waveform can be obtained by the following expression 3.

  When the above equation 3 is substituted into the above equation 2, the absorption waveform y (t) is expressed by the following equation 4.

  Here, in order to obtain y (t), the inverse problem of what laser light wavelength (λ (t)) should be given is considered. That is, the following equation 5 is considered with y (t) = φ (λ).

  The above equation 5 is expanded as the following equation 6.

  That is, from the above equation 6, it can be seen that the wavelength of the laser light may be modulated as in the following equation 7.

  Since the wavelength of the laser light depends on the intensity of the modulated wave signal, the waveform of the modulated wave signal is expressed by the following formula 8 with λ replaced by the intensity x.

That is, the use of the modulated wave signal represented by the above formula 8, the absorption waveform is a sine wave at time t ₀ the vicinity. The waveform shown in FIG. 5 shows the waveform of the above equation 8.

  In the above equation 8, the value of the amplitude coefficient δ is preferably 0.005 or more and 0.05 or less. This will be described below.

FIG. 8 shows the relationship between the amplitude coefficient δ, the amplitude of the absorption waveform, and the abundance ratio (spectral ratio) of the harmonic component included in the modulated wave, assuming that the absorption factor = 1 at the absorption center wavelength λ ₀ . Show. In FIG. 8, the amplitude 22 of the absorption waveform, the amplitude 23 of the third harmonic component (fundamental wave ratio), and the amplitude 24 of the fifth harmonic component (fundamental wave ratio) are shown. In FIG. 8, the amplitude coefficient δ is shown in a range from 0.0001 to 1. As shown in FIG. 8, the amplitude of the absorption waveform increases as the amplitude coefficient δ decreases. If the amplitude of the absorption waveform is large, the absorption of the laser beam can be detected with high sensitivity. Therefore, it is preferable that the amplitude coefficient δ is small from the viewpoint of performing detection with high sensitivity.

  On the other hand, the higher the amplitude coefficient δ, the smaller the spectral ratio of the harmonic components contained in the modulated wave. 9 to 17 are graphs showing (a) the waveform of the modulated wave signal and (b) the absorption waveform, respectively. The frequency of the modulated wave signal is 5 kHz. 9 is δ = 0.9, FIG. 10 is δ = 0.5, FIG. 11 is δ = 0.1, FIG. 12 is δ = 0.05, FIG. 13 is δ = 0.01, FIG. FIG. 15 shows the waveform when δ = 0.001, FIG. 16 shows the waveform when δ = 0.0005, and FIG. 17 shows the waveform when δ = 0.0001. As shown in FIGS. 9 to 17, the waveform of the modulated wave signal shows a sharp peak as δ decreases. This indicates that the harmonic components of 3 times and 5 times are mainly increased. On the other hand, FIGS. 18 to 26 show the results of Fourier transform analysis of the waveforms shown in FIGS. 18 is δ = 0.9, FIG. 19 is δ = 0.5, FIG. 20 is δ = 0.1, FIG. 21 is δ = 0.05, FIG. 22 is δ = 0.01, and FIG. FIG. 24 shows the results of Fourier transform analysis when δ = 0.005, FIG. 24 shows δ = 0.001, FIG. 25 shows δ = 0.0005, and FIG. 26 shows δ = 0.0001. 18 to 26, it is confirmed that the peak of the harmonic component (15 (kHz)) is smaller as δ is larger from the Fourier transform analysis result of the modulated wave. From the viewpoint of reducing the abundance ratio of harmonic components, it is preferable that the ratio is large, and from the results of Fourier transform analysis of the absorption waveforms shown in FIGS. It can be seen that this is a pure sine waveform.

  As described above, the preferable value of the amplitude coefficient δ has a trade-off relationship between increasing the amplitude of the absorption waveform and suppressing the harmonic component. Here, if the amplitude coefficient δ is in the range of 0.005 to 0.05, the amplitude of the absorption waveform can be increased while sufficiently suppressing the harmonic component as shown in FIG. .

  The amplitude coefficient δ is more preferably 0.01. As shown in FIG. 8, when the amplitude coefficient δ is set to 0.01, the ratio of the third harmonic component in the modulated wave signal is 3.8%, and the ratio of the fifth harmonic component is 0.7. %. That is, it is possible to sufficiently reduce the harmonic component compared to the fundamental wave component. Furthermore, as shown in FIG. 22A, it is confirmed that the harmonic component (15 kHz) of the modulated wave signal when δ = 0.01 is sufficiently smaller than the fundamental wave component. The

FIG. 27 shows the relationship between the amplitude coefficient δ and the amplitude of the modulated wave signal. When the amplitude coefficient δ is set to 0.01, the amplitude of the modulated wave signal is 3.03. Here, since θ (λ) follows a Gaussian distribution, λ when θ (λ) = 0.5 is λ ₀ ± 1.18. Therefore, the full width at half maximum is 2.38 because 2 × 1.18 = 2.36. That is, when the amplitude coefficient δ is set to 0.01, the amplitude of the modulated wave is 3.03 / 2.36 = 1.28, which is 1.28 times the half-value width. Thus, when δ = 0.01 is set, it is also preferable from the viewpoint of obtaining an amplitude sufficiently larger than the half width.

  FIG. 28 shows a simulation result when a modulated wave signal having a waveform represented by Expression 8 is used. The amplitude coefficient δ is set to 0.01. In FIG. 28, (a) shows the waveform of the modulated wave signal (B1 or B2), (b) shows the waveform of the synthesized signal C, (c) shows the waveform of the absorption signal F, and (d) shows the demodulation. The waveform of the signal (G1 or G2) is shown, and (e) shows the waveform of the low frequency component signal (H1 or H2). In FIG. 28, the time when the center wavelength of the laser light coincides with the absorption wavelength of the measurement target is 0.045 (s). As shown in FIG. 28C, it is confirmed that the waveform of the absorption signal F has a sine wave shape around the time 0.045. As shown in FIG. 28E, the peak value of the low frequency component signal (H1 or H2) after being processed by the low pass filter was 2217 (A / D value).

FIG. 29 shows a simulation result when the wavelength of the laser beam is modulated by a sine wave for comparison. As in FIG. 28, in FIG. 29, (a) shows the waveform of the sine wave signal, (b) shows the waveform of the composite signal C, (c) shows the waveform of the absorption signal F, and (d) shows The waveform of the demodulated signal is shown, and (e) shows the waveform of the low frequency component signal. Similarly to FIG. 28, the time when the center wavelength of the laser light coincides with the absorption wavelength λ ₀ of the measurement target is 0.045 (s). As shown in FIG. 29 (c), the width of the waveform of the absorption signal F is narrow around time 0.045. In the low frequency component signal (H1 or H2) after being processed by the low pass filter, the peak value was 916 (A / D value).

  Comparing FIG. 28 (e) and FIG. 29 (e), by using the modulated wave signal according to the present embodiment, the peak value is 2.3 times (= 2217/916) when using a sine wave. It can be seen that absorption of laser light by the measurement object can be detected with high sensitivity.

  FIG. 30 shows a simulation result when the wavelength of the laser beam is modulated by a sine wave having an optimized amplitude. As in FIG. 28, in FIG. 30, (a) shows the waveform of the sine wave signal, (b) shows the waveform of the composite signal C, (c) shows the waveform of the absorption signal F, and (d) shows The waveform of the demodulated signal is shown, and (e) shows the waveform of the low frequency component signal. As shown in FIG. 30 (e), the peak value of the low frequency component signal (H1 or H2) was 1465 (A / D value). Thus, when compared with the peak value (916) of the waveform shown in FIG. 29 (e), it can be seen that the peak value can also be increased by optimizing the amplitude. However, as shown in FIG. 28 (e), when the modulated wave signal according to the present embodiment is used, the peak value can be further increased compared with the case where the amplitude of the sine wave is optimized, and the effect is further improved. You can see that

  As described above, according to the present embodiment, the wavelength of the laser beam is modulated with a waveform smaller than that of a sine wave having the same amplitude and the same frequency with the same absolute value of inclination when the intensity becomes zero. Therefore, it is possible to detect the absorption of the laser beam by the measurement object with high sensitivity.

  In the present embodiment, the case where two different frequency signals (the first modulated wave signal B1 and the second modulated wave signal B2) are used as the modulated wave signal has been described. From the viewpoint of detecting the absorption of laser light with higher sensitivity, both of the two modulated wave signals are smaller in absolute value of the slope when the intensity becomes zero than that of a sine wave having the same amplitude and the same frequency. A waveform is preferred. However, even if the modulation wave signal according to the present embodiment is applied to only one of the two modulation wave signals, the absorption of the laser beam can be detected with higher sensitivity than when a sine wave is used.

  Further, it is not always necessary to prepare two signals having different frequencies as the modulation wave signal, and the present invention can be applied even when there is only one modulation wave signal.

  In the present embodiment, the differential amplifier 12 is provided in the light receiving processing unit 3, and the amplified signal E is differentially amplified by the combined signal C by the differential amplifier to generate the absorption signal F. However, the configuration of the light reception processing unit 3 described in the present embodiment is merely an example, and other configurations may be employed. For example, the absorption signal F indicating the amount of laser light absorption may be generated by linearly complementing the amplified signal E without using the synthesized signal C. Even in such a case, it is possible to achieve an effect by using the modulated wave signal according to the present embodiment.

  Moreover, in this embodiment, the waveform shown by Formula 8 was illustrated as a preferable waveform of a modulated wave signal. However, if the absolute value of the slope when the intensity becomes zero is a waveform smaller than that of a sine wave having the same amplitude and the same frequency, it is possible to use a waveform different from the waveform shown in Equation 8. is there.

  For example, a waveform represented by the cube of a sine wave can be used as the waveform of the modulated wave signal. FIG. 31 is a graph showing a waveform indicated by the cube of a sine wave. In FIG. 31, the horizontal axis indicates time t, and the vertical axis indicates signal intensity. In FIG. 31, a sine wave is shown by a dotted line for comparison. As shown in FIG. 31, the waveform indicated by the cube of the sine wave also has a smaller absolute value of the slope when the intensity becomes zero than that of a sine wave having the same amplitude and the same frequency.

  FIG. 32 shows a simulation result when a waveform represented by the cube of a sine wave is used as the waveform of the modulated wave signal. As in FIG. 28, in FIG. 32, (a) shows the waveform of the modulated wave signal, (b) shows the waveform of the synthesized signal C, (c) shows the waveform of the absorption signal F, and (d) shows The waveform of the demodulated signal is shown, and (e) shows the waveform of the low frequency component signal. As shown in FIG. 32 (e), the peak value of the low-frequency component signal (H1 or H2) is 2149 (A / D value), which is more than when a sine wave is used (see FIGS. 29 and 30). A large value was obtained.

(Second Embodiment)
Subsequently, a second embodiment of the present invention will be described. FIG. 33 is a block diagram showing the concentration measuring apparatus 1 according to this embodiment. In the present embodiment, the configuration of the light reception processing unit 3 is devised with respect to the first embodiment. Specifically, a first function processor 20 and a second function processor 21 are added to the light reception processing unit 3. Unlike the first embodiment, the first modulated wave generator 8-1 and the second modulated wave generator 8-2 generate a sinusoidal modulated wave signal. Since other points can be the same as those in the first embodiment, a detailed description thereof will be omitted.

  In the present embodiment, the wavelength of the laser light is modulated with a sine wave. Therefore, in the light receiving processing unit 3, a signal (absorption signal F) indicating the amount of laser light absorbed by the measurement target has a waveform as shown in FIG. In this state, as described in the first embodiment, the peak value of the signal after passing through the low-pass filter does not increase. Therefore, processing is performed by the first function processor 20 and the second function processor 21 so that the waveform of the absorption signal F becomes a sine wave.

  The first function processor 20 is provided between the analog / digital converter 14 and the first phase detector 15. The second function processor 21 is provided between the first low-pass filter 16 and the second phase detector 17. The first function processor 20 is provided corresponding to the first modulated wave signal B1, and the second function processor 21 is provided corresponding to the second modulated wave signal B2. Since the substantial processing contents of the first function processor 20 and the second function processor 21 are the same, only the processing of the first function processor 20 will be described below.

  The first function processor 20 multiplies the absorption signal F (hereinafter referred to as a digital absorption signal) converted into digital data by the analog / digital converter 14 by a function represented by the following equation (9).

  In the above equation 9, δ represents the amplitude coefficient, and λ represents the wavelength of the laser beam. Since the wavelength of the laser beam is indicated by a sine wave, λ (t) is expressed by the following formula 10.

  In Equation 10, a represents the amplitude of the sine wave used as the first modulated wave B1, and ω represents the angular frequency of the sine wave used for modulation.

In Expression 9, denominator exp (−½ × λ ² (t)) represents the absorption signal F. Therefore, the signal converted by the function processor 20 (referred to as a converted absorption signal) becomes a sine waveform represented by the numerator in Equation 9.

  Thereafter, the converted absorption signal generated by the first function processor 20 is phase-detected by the first phase detector 15 and then the low-frequency component is extracted by the first low-pass filter 16. Since the waveform of the converted absorption signal is a sine wave, the peak value of the signal after passing through the low-pass filter can be increased as described in the first embodiment, and the laser beam of the measurement target can be increased. The amount of absorption can be detected with high sensitivity.

  In this embodiment, the function processor (20, 21) is provided at the subsequent stage of the analog / digital converter 14. Accordingly, the converted absorption signal having a sine waveform is obtained by digital processing. A sine wave is used as the modulation wave signal. The signal as shown in Expression 11 of the first embodiment has a point where the slope changes abruptly (see FIG. 5). When a modulated wave signal having such a waveform is to be generated, the generated modulated wave signal is likely to contain harmonic components. On the other hand, if a sine wave is used as the modulation wave signal, harmonic components can be suppressed, and adverse effects due to noise generation can be eliminated.

  The first and second embodiments have been described above. However, these embodiments are not independent from each other, and can be used in combination within a consistent range.

DESCRIPTION OF SYMBOLS 1 Gas concentration measuring device 2 Light emission control part 3 Light reception process part 4 Laser diode 5 Gas object area | region 6 Photodiode 7 Carrier wave generator 8-1 1st modulation wave generator 8-2 2nd modulation wave generator 9 Adder 10 Laser diode driver 11 Photodiode amplifier 12 Differential amplifier 13 Gain amplifier 14 Analog-digital converter 15 First phase detector 16 First low-pass filter 17 Second phase detector 18 Second low-pass filter 19 Gas concentration calculator 20 First Function processor 21 Second function processor 22 Absorption waveform amplitude 23 3rd harmonic component 24 5th harmonic component A Carrier wave signal B1 First modulated wave signal B2 Second modulated wave signal C Composite signal D Light received signal E Measurement signal F Absorption signal G1 First demodulated signal G2 Second demodulated signal H1 First low frequency component signal H2 Second low Wave component signal

Claims (25)

A light emission control unit that controls a laser diode that emits laser light toward a measurement target so that the wavelength of the laser light is modulated by a first modulated wave;
A light reception processing unit that acquires a light reception signal from a photodiode that receives the laser light that has passed through the measurement target, and measures the concentration of the measurement target based on the light reception signal;
Comprising
The concentration measuring apparatus, wherein the first modulated wave has a waveform whose absolute value of inclination when the intensity becomes zero is smaller than that of a sine wave having the same amplitude and the same frequency. The concentration measuring device according to claim 1,
The frequency of the first modulated wave is f,
The light receiving processing unit
A phase detector that detects a component having a frequency of 2 × f based on the received light signal and generates a demodulated signal;
A low-pass filter that extracts a low-frequency component of the demodulated signal and generates a low-frequency component signal;
A concentration measurement apparatus comprising: a concentration calculation unit that calculates the concentration of the measurement target based on the low frequency component signal. A concentration measuring device according to claim 2,
The light reception processing unit further includes a differential amplifier that generates an absorption signal indicating an absorption amount of the laser light by the measurement target by comparing the light reception signal with a reference signal,
The phase detector is a concentration measurement device that generates the demodulated signal based on the absorption signal. A concentration measuring device according to any one of claims 1 to 3,
The concentration measuring apparatus is such that the waveform of the first modulated wave is a waveform such that a temporal change in the amount of absorption of the laser light by the measurement target becomes a sine wave. The concentration measuring device according to claim 4,
When the amount of light absorption by the measurement object shows a Gaussian distribution with respect to the wavelength,
The waveform of the first modulated wave is expressed by the following formula 1.

In Equation 1, a concentration measuring device in which t is time, λ is wavelength, T is period, n is a natural number, and δ is an amplitude coefficient. The concentration measuring device according to claim 5,
The concentration measurement device in which δ is 0.005 or more and 0.05 or less. A concentration measuring device according to any one of claims 1 to 3,
The concentration measuring device, wherein the waveform of the first modulated wave is a waveform represented by the third power of a sine wave. A concentration measuring device according to any one of claims 1 to 7,
The light emission control unit
A carrier generator for generating a carrier signal;
A first modulated wave generator for generating a first modulated wave signal having a waveform corresponding to the first modulated wave;
An adder for adding the carrier signal and the first modulated wave signal to generate a composite signal;
A concentration measurement apparatus comprising: a laser diode driver that controls an injection current amount for the laser diode based on the synthesized signal. The concentration measuring device according to claim 8,
The carrier wave signal is a concentration measuring device which is a ramp wave. A concentration measuring device according to claim 8 or 9, wherein
The light emission control unit further includes a second modulation wave generator that generates a second modulation wave signal having a frequency different from that of the first modulation wave,
The adder generates a signal obtained by adding the carrier wave signal, the first modulated wave signal, and the second modulated wave signal as the synthesized signal,
The second modulated wave signal generator, as the second modulated wave signal, is a sinusoidal wave having the same amplitude and the same frequency as that of the first modulated wave. A concentration measurement device that generates a signal having a smaller waveform. A concentration measuring device according to any one of claims 1 to 10,
The measurement object is a concentration measuring device that is a gas component. A light emission control unit that controls a laser diode that emits laser light toward a measurement target so that the wavelength of the laser light is modulated by a sine wave;
A learning signal is obtained from a photodiode that receives the laser beam that has passed through the measurement target, and based on the light reception signal, a light reception processing unit that measures the concentration of the measurement target;
Comprising
The light receiving processing unit
Based on the received signal, an absorption signal generation unit that generates an absorption signal indicating the amount of absorption of the laser light by the measurement target;
A concentration measurement apparatus comprising: a function processor that converts the absorption signal using a processing function so that a waveform becomes a sine wave and generates a converted absorption signal. A concentration measuring device according to claim 12,
The function processor generates the converted absorption signal by multiplying the absorption signal by a function f (t) expressed by Equation 2 below.

In Equation 2, t is the time, λ is the wavelength of the laser beam, δ is the amplitude coefficient, and n is a concentration measuring device indicating a natural number. The concentration measuring device according to claim 12 or 13,
The light receiving processing unit
An analog / digital converter that converts the absorption signal into a digital signal to generate a digital absorption signal;
The function processor is a concentration measurement device that performs processing using the processing function on the digital absorption signal. Controlling a laser diode that emits laser light toward a measurement target so that the wavelength of the laser light is modulated by a first modulated wave;
Receiving the laser beam transmitted through the measurement object and generating a light reception signal;
Measuring the concentration of the measurement object based on the light reception signal;
Comprising
The concentration measurement method, wherein the first modulated wave has a waveform whose absolute value of inclination when the intensity becomes zero is smaller than that of a sine wave having the same amplitude and the same frequency. The concentration measurement method according to claim 15, comprising:
The frequency of the first modulated wave is f,
The step of measuring the concentration comprises:
Detecting a component having a frequency of 2 × f based on the received light signal and generating a demodulated signal;
Extracting a low frequency component of the demodulated signal to generate a low frequency component signal;
A concentration measuring method comprising: calculating a concentration of the measurement target based on the low frequency component signal. A concentration measuring method according to claim 16, comprising:
The step of measuring the concentration further comprises:
Generating an absorption signal indicating the amount of absorption of the laser beam by the measurement object by comparing the light reception signal with a reference signal;
The concentration measurement method, wherein the step of generating the demodulated signal includes the step of generating the demodulated signal based on the absorption signal. A concentration measurement method according to any one of claims 15 to 17,
When the amount of light absorption by the measurement object shows a Gaussian distribution with respect to the wavelength,
The concentration measurement method, wherein the first modulated wave is a waveform such that a temporal change in the amount of absorption of the laser light by the measurement target becomes a sine wave. The concentration measurement method according to claim 18, comprising:
The waveform λ (t) of the first modulated wave is expressed by the following formula 1.

In Equation 3, the concentration measurement method in which t is time, λ is wavelength, T is period, n is a natural number, and δ is an amplitude coefficient. It is the density | concentration measuring method described in Claim 19, Comprising:
The concentration measurement method, wherein δ is 0.005 or more and 0.05 or less. A concentration measurement method according to any one of claims 15 to 17,
The concentration measurement method, wherein the waveform of the first modulated wave is a waveform represented by a cube of a sine wave. The concentration measuring method according to any one of claims 15 to 21,
The controlling step includes
Generating a carrier wave signal;
Generating a first modulated wave signal having a waveform corresponding to the first modulated wave;
Adding the carrier signal and the first modulated wave signal to generate a composite signal;
And a step of controlling an injection current amount for the laser diode based on the synthesized signal. It is the density | concentration measuring method described in Claim 22, Comprising:
The step of generating the carrier signal includes the step of generating a ramp wave as the carrier signal. The concentration measurement method according to claim 22 or 23, wherein:
The step of controlling further comprises the step of generating a second modulated wave signal with a waveform having a frequency different from that of the first modulated wave,
The step of generating the combined signal includes the step of generating a signal obtained by adding the carrier signal, the first modulated wave signal, and the second modulated wave signal as the combined signal,
Similar to the first modulated wave, the waveform of the second modulated wave signal is a waveform in which the absolute value of the slope when the intensity becomes zero is smaller than that of a sine wave having the same amplitude and the same frequency. Concentration measurement method. A concentration measuring method according to any one of claims 15 to 24, wherein:
The measurement object is a concentration measurement method that is a gas component.

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